Metallic mirrors have numerous applications. In the field of optics, for example, metallic mirrors are employed to direct and focus electromagnetic radiation. Mirror surfaces are traditionally formed by applying a metallic coating to a substrate composed of another metal or of another material such as glass. When subjected to temperature variations, such as those occurring in various space and military applications, differential rates of expansion and contraction of the joined materials typically lead to dimensional instability of the mirror surface. One solution has been to create a mirror surface composed of a single metal or metal alloy, where the mirror surface is generated by polishing a surface of the single-component object. Use of a single metal or metal alloy in a single-component mirror surface obviates problems caused by differences in thermal properties of joined materials and provides enhanced mechanical strength and rigidity. Aluminum is a suitable metal in a single-component mirror surface due to its high reflectivity, light weight, low cost, and compatibility with conventional surface forming processes. However, aluminum is a relatively soft material with a Mohs hardness of 2.75. As such, use of conventional polishing abrasives with a Mohs hardness of about 8.5 and above (e.g., alumina or silicon carbide) may result in scratches or chatter marks on the aluminum mirror surface.
The performance of such mirrors in optics applications depends strongly on the mirror surface being highly uniform to minimize light scattering caused by topographical variations and defects. To optimize performance of the mirror surface, the topographical variations and defects at the surface of the mirror must be reduced or eliminated. Surface roughness parameters relevant to optical performance include not only average surface roughness (Ra) or root mean square surface roughness (Rq), but also importantly parameters including Rmax and/or Rz. Rmax is the largest peak-to-valley height in a given sampling region wherein the peak represents a high spot on the mirror surface and the valley represents the depth of a scratch or other topographical variation on the mirror surface. Rz is an average value for Rmax measured in several distinct sampling regions. Not only must average surface roughness be low for good optical performance, but also Rz, Rmax or related parameters must be low to minimize light scattering.
Highly-polished aluminum mirror surfaces have additional applications in the field of solar cell fabrication. Mirror arrays are frequently used to concentrate solar radiation onto photovoltaic solar cells to improve conversion efficiency. Because diffraction of light by surface irregularities on the mirror surfaces results in lowered efficiencies in conversion of incident solar radiation to electric power, improved methods for economically producing highly-reflective mirror surfaces composed of aluminum can facilitate development of solar cell technology.
Two methods commonly used to polish aluminum surfaces are single-point diamond turning and lapping. In the field of optics, single-point diamond turning has been used for many years to produce aluminum mirrors useful for reflecting infrared (e.g., long wavelength) light. In single-point diamond turning, an aluminum substrate is rotated while in contact with a precisely positioned diamond cutting tool. The diamond cutting tool “peels” a very thin layer of aluminum from the surface of the substrate to form a surface having a precisely defined geometry. The diamond turning technique typically can achieve surface roughness (Rq) around 50 Å. However, for optical applications, a lower surface roughness (Rq) is required, such as about 20 Å. In addition, the diamond cutting tool produces microscopic grooves on the surface of the substrate that compromise optical performance due to light scattering, particularly at shorter wavelengths. Further, single-point diamond turning is an expensive and highly time-consuming process suitable only for low volume production of specialized optical components.
The most commonly used process for polishing aluminum surfaces is lapping. In lapping, a slurry of abrasive particles is used to polish an aluminum surface by moving a polishing surface relative to the aluminum surface with the abrasive slurry therebetween. The abrasive slurry is commonly a suspension of aluminum oxide or silicon carbide particles in a carrier of water or oil. The aluminum surface is abraded by mechanical action of the particles in the abrasive slurry. However, lapping generates shavings of aluminum from the aluminum surface that tend to produce microscratches that are unacceptable for optical applications. In addition, abrasive slurries that contain a plurality of abrasive particles suspended in liquid carrier are often colloidally unstable. In particular, the abrasive particles tend to settle out of the liquid carrier, agglomerate, or aggregate, any of which may result in irregularities on the mirror surface.
Chemical-mechanical polishing or planarization (CMP) has long been used in the electronics industry to polish or planarize the surface of memory or rigid disks. Typically, memory or rigid disks comprise an aluminum substrate coated with a first layer of nickel-phosphorus. The nickel-phosphorus layer is frequently planarized by a CMP process to reduce surface roughness prior to coating with a magnetic layer, such as cobalt-phosphorus. However, CMP may produce microscratches and leave imbedded abrasive particles on the polished surface. These defects may be acceptable for the noted electronics applications, but cannot be tolerated in optical applications.
Thus, there remains a need for efficient and economical methods of polishing aluminum surfaces to exacting standards of surface roughness suitable for at least substantially diffraction-free reflectance of light in the visible and ultraviolet ranges. The invention provides such a method. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.